Sliding wall variable geometry turbines are commonly provided with a piston ring, to mitigate passage of exhaust gas between the main gas passage (from the volute through the nozzle to the turbine) and a region behind the nozzle. Gas flow through leakage paths is inherently inefficient and is disadvantageous in standard operating conditions. The relatively high temperature environment requires clearances to be incorporated between moving parts so that they do not rub together and wear or jam. The exhaust gas temperatures generally limit the ability to use relatively low temperature capable materials for the piston ring.
The clearance between vanes and a shroud (where provided) cannot generally be provided with a positive seal; however, sliding wall variable geometry turbine mechanisms typically have a cylindrical element which slides from or retracts into a corresponding cylindrical cavity. The clearance between these two cylindrical elements can be sealed quite effectively with a piston ring, which is typically retained in a groove in one of the elements, and is resiliently biased towards the other element.
In some prior sliding wall variable geometry turbine mechanisms the piston ring is retained in the radially outboard, inwardly facing surface of an annular cavity, and is resiliently biased radially inwardly to rest against the surface of an outer cylindrical wall of a sliding gas control surface (which might include a radial wall with vanes or a shroud). This piston ring may be retained in a groove defined in the interface between the bearing housing and the turbine housing.
An engineer designing a turbocharger often considers many factors including the cost of the turbocharger, the ease of packaging the turbocharger into an engine compartment, as well as the width of the shaft between the turbine and compressor which affects efficiency of fluid film bearings if provided. One consideration is to minimize the axial length of the turbocharger while recognizing that the piston ring is axially near the main gas passage (i.e. axially outboard away from the compressor and bearing housings). Conversely the V-band, which retains the turbine and bearing housings together, is positioned at an axial distance inboard of the turbine housing, due to the width of the V-band.
The technical field of gas sealing arrangements disposed about a sliding wall of a variable geometry turbine of a turbocharger is an area where further technical developments are desired. The present application contributes to this technical field in a novel and nonobvious manner.